Communication devices such as terminals are also known as e.g. User Equipments (UE), mobile terminals, stations (STAs), wireless devices, wireless terminals and/or mobile stations. Communications devices are enabled to communicate wirelessly in a wireless communications network, such as a Wireless Local Area Network (WLAN), or a cellular communications network sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two communications devices, between a communications device and a regular telephone and/or between a communications device and a server via an access network and possibly one or more core networks, comprised within the wireless communications network.
The above communications devices may further be referred to as mobile telephones, cellular telephones, laptops, or tablets with wireless capability, just to mention some further examples. The communications devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the access network, such as a Radio Access Network (RAN), with another entity, such as another terminal or a server.
The communications network covers a geographical area which is divided into geographical subareas, such as coverage areas, cells or clusters. In a cellular communications network each cell area is served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or Base Transceiver Station (BTS), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB, micro eNode B or pico base station, based on transmission power, functional capabilities and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the communications devices within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the communications device. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the communications device to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
Location-based services and emergency call positioning drive the development of positioning in the wireless communications networks. A positioning support in the Third Generation Partnership Project Long Term Evolution (3GPP LTE) was introduced in Release 9. This positioning support enables operators to retrieve position information for location-based services and to meet regulatory emergency call positioning requirements.
Positioning in an LTE communications network is supported by a network architecture schematically illustrated in FIG. 1. As illustrated, the network architecture supports direct interactions between a communications device, e.g. a UE, and a network node, e.g. a location server such as an Evolved-Serving Mobile Location Centre (E-SMLC), via a positioning protocol such as an LTE Positioning Protocol (LPP). Moreover, the network architecture supports interactions between the location server, e.g. the E-SM LC, and a Radio Network Node, e.g. an eNodeB, via an LPPa protocol. The interactions between the E-SMLC and the eNodeB may to some extent be supported by interactions between the eNodeB and the UE via a Radio Resource Control (RRC) protocol.
Four positioning techniques are considered in LTE, and they will be briefly described below.
A first positioning technique is based on an Enhanced Cell ID. Essentially, cell ID information is used to associate the communications device to a serving area of a serving cell, and then additional information is used to determine a finer granularity position.
A second positioning technique is based on an Assisted Global Navigation Satellite System (GNSS). GNSS information is retrieved by the communications device, and is supported by assistance information provided to the communications device from the E-SM LC to determine a position of the communications device.
A third positioning technique is based on an Observed Time Difference of Arrival (OTDOA). The communications device estimates a time difference of reference signals received from different eNodeBs and sends the estimated time differences to the E-SM LC for multilateration.
A fourth positioning technique is based on an Uplink Time Difference of Arrival (UTDOA). The communications device is requested to transmit a specific waveform that is detected by multiple location measurement units, e.g. multiple eNodeBs, at known positions. These measurements are forwarded to the E-SMLC for multilateration.
Global Positioning System (GPS)-enabled communications devices may meet the requirement for positioning, but the GPS may not provide the required availability due to the satellite signals being blocked in urban and/or indoor environments. Therefore, other techniques are needed in such environments. The OTDOA has been introduced in the 3GPP release 9 as a downlink (DL) positioning method. As schematically illustrated in FIG. 2, the OTDOA in LTE is based on the communications device measuring the Time Of Arrival (TOA) of signals received from the eNodeBs. The communications device measures the relative difference between a reference cell and another specific cell, defined as a Reference Signal Time Difference (RSTD) measurement. Every such RSTD determines a hyperbola and the intersection point of these hyperbolas may be considered as the position of the communications device. Here, the reference cell is selected by the communications device and the RSTD measurement may be performed on an intra-frequency cell or on an inter-frequency cell. By the expression “an intra-frequency cell” when used herein is meant that the reference cell and/or a neighbour cell is on the same carrier frequency as the serving cell. Further, by the expression “an inter-frequency cell” when used in this disclosure is meant that at least one of reference cell and the neighbour cell is on a different carrier frequency from the serving cell.
It is possible to measure the RSTD on any downlink signals e.g., on a Cell-specific Reference Signal (CRS). However, in the OTDOA the communications device is required to detect multiple neighbour-cell signals, and these signals may suffer from poor hearability. Hence, Positioning Reference Signals (PRSs) have been introduced to improve the OTDOA positioning performance.
FIGS. 3A and 3B schematically show the arrangement of the PRS assigned resource elements, denoted with R6, for PRS subframes using a normal Cyclic Prefix (CP). Further, the FIGS. 3A and 3B schematically show the mapping using one and two Physical Broadcast Channel (PBCH) antenna ports and four PBCH antenna ports, respectively. As illustrated in FIGS. 3A and 3B, the number of PRS resources in a PRS subframe vary depending on the configuration of the number of PBCH antenna ports and what cyclic prefix is used.
FIGS. 4A and 4B schematically show the arrangement of the PRS assigned resource elements, denoted with R6, for PRS subframes using an extended CP. Further, the FIGS. 4A and 4B schematically show the mapping using one and two PBCH antenna ports and four PBCH antenna ports, respectively. As illustrated in FIGS. 4A and 4B, the number of PRS resources in a PRS subframe vary depending on the configuration of the number of PBCH antenna ports and what cyclic prefix is used.
A PRS subframe may comprise positioning reference signal, downlink control information, and Cell specific Reference Signal (CRS). However, the PRS subframe does not comprise any data symbols. Basically, the resource elements that may be used for data transmission are left unused in the PRS subframe in order to reduce interference between different networks during transmission of the PRS subframes. This improves hearability, i.e., the PRS may be heard or detected more easily if there is less interference.
Thus, in order to reduce the interference with neighbour cells in the PRS subframe, no Physical Downlink Shared Channel (PDSCH) data is carried. The Physical Downlink Control Channel (PDCCH) and the CRSs are retained in the PRS subframe, while PRSs R6 are distributed in a “diagonal” way in between the CRSs. In FIGS. 3A, 3B and 4A, 4B the CRSs are not shown, although they are always present in PRS subframes. Similar to the CRS, a cell-specific frequency shift is applied to the PRS pattern, thereby avoiding time-frequency PRS collisions with up to six neighbours cells. The cell-specific frequency shift may be given by a Physical Cell ID (PCI) modulo 6.
In an LTE system, two or more consecutive PRS subframes, a.k.a. positioning occasions, are transmitted periodically in the downlink. One positioning occasion may comprise up to six consecutive PRS subframes. The time period of one positioning occasion may be configured to every TPRS=160, 320, 640 and 1280 milliseconds. It should be noted that, in a Time Division Duplexing (TDD) mode, an uplink subframe and other special frames may not comprise PRSs. Another parameter to characterize the PRS transmission schedule is the cell specific subframe offset, which defines the starting subframe of the PRS transmission relative to a Subframe Number (SFN) equal to 0. As shown in Table 1 below, the PRS periodicity TPRS and the subframe offset ΔPRS are derived from the PRS Configuration Index IPRS.
TABLE 1PRS subframe configurationPRS configurationPRS periodicity TPRSPRS subframe offsetIndex IPRS(subframes)ΔPRS (subframes) 0-159160IPRS160-479320IPRS-160 480-1119640IPRS-4801120-23991280 IPRS-11202400-4095Reserved
In some cases, and in particular in cases of dense deployment, it may not be sufficient to only use cell-specific frequency shift in order to avoid interference from neighbour cells. Therefore, PRS muting has been introduced to further reduce inter-cell interference by muting PRS transmission in other cells based on a periodical muting pattern. By the expression “dense deployment” when used in this disclosure is meant that a large number of base stations, e.g. eNBs, are present in a certain geographical area. Note that only PRS of up to six base stations may use orthogonal time and/or frequency resources according to the PRS specified in LTE, if muting is not used. The PRS muting configuration of a cell is defined by a periodic muting sequence with a periodicity of TREP, where TREP counted in number of PRS positioning occasions may be 2, 4, 8, or 16. Correspondingly, the PRS configuration is represented by a bit string of length 2, 4, 8, or 16.
Use of multi-antenna techniques may increase the cell hearability. By the expression “cell hearability” when used in this disclosure is meant an improved detectability of the PRS transmitted from a cell, e.g. a base station, and received by a communications device, e.g. a UE. A PRS transmitted from the base station will be unheard if it is not detected by the communications device, i.e. when the signal quality of the PRS transmitted from the base station is insufficient. A PRS will be heard poorly if it is detected but the received PRS is very noisy, which would result in an inaccurate Time of Arrival (TOA) estimate. By spreading the total transmission power wisely over multiple transmit antennas, an array gain may be achieved which increases the signal quality at a receiver, e.g. the communications device. The transmitted signal from each transmit antenna is formed in such way that the received signal from each antenna adds up coherently at the receiver. This is referred to as beam-form ing. Precoding describes how to form each antenna in the antenna array in order to form a beam. In other words, precoding controls the phase and relative amplitude of the signal at each transmit antenna in order to create a pattern of constructive and destructive interference in the wavefront. Precoding may be separated into analog and digital precoding. Analog precoding means that antenna elements are combined with connecting circuity to form a physical antenna. Thus, the mapping between the input of the physical antenna to the different antenna elements is referred to as an analog precoder. Digital precoding means that weights may be assigned to map signal components from logical antenna ports to the physical antennas. A codebook is a set of precoding vectors, each of which precoding vector is used to control the transmit antennas in the antenna array to form the beam. Sometimes the precoding vectors are referred to as beam-forming vectors. Typically, the codebook lists precoding vectors for digital precoding, while the analog precoding is designed as part of the design of the physical antenna. One digital precoder alternative is based on a selection of an analog precoder, which means that the digital precoding vector comprises a 1 corresponding to the selected physical antenna, and 0 for the rest of the available physical antennas. In general, general digital precoding weights can be considered between the antenna ports and the physical antennas.
A different perspective is to consider the combination of a digital and an analog precoder and corresponding codebooks as one precoder with a corresponding codebook. In this case, there is one joint mapping from each antenna port to each individual antenna elements, and one codebook listing all available mappings between antenna ports and antenna elements.
In an LTE positioning system, a cell-specific frequency shift, for example given by PCI modulo 6, i.e. mod(PCI,6), is applied to a PRS pattern, which helps to avoid time-frequency PRS collision from up to six neighbour cells. However, even in a properly planned communications network in respect to mod(PCI,6), two or more cells will always create interference towards each other since only up to six cells may use orthogonal time and/or frequency resources for the transmission of PRS according to the current LTE specification, and this interference is static besides a fast-fading component which is due to the time variant nature of the channel between the base station and the communications device. In other words, the fast-fading component may due to that the channel between the base station and the communications device may vary over time. Consequently, a neighbour cell may not be heard due to a colliding PRS pattern, and this may be the case for the complete positioning measurement time, e.g. for an LPP response time period. By removing the static property of the interference, the probability of more hearable neighbour cells during the complete positioning measurement time increases. By the expressions “interference is static” and “static interference” when used in this disclosure is meant that the interference situation at the communications device is not changing over time. If the interference is not static, there will be some instances with lower interference and some instances with higher interference. The instances with lower interference will provide an opportunity to receive PRS with less interference, leading to an improved TOA estimation. One way of reducing the interference from colliding PRS patterns is to apply a muting pattern. However, muting leads to resource waste in the sense that some time and/or frequency resources are not used for any PRS or data transmission in the muted cell.
Further, in the prior art networks the PRS may be transmitted in a direction advantageous for the performance of some communications devices while being disadvantageous for the performance of other communications devices.